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Reaction mechanisms

Most reactions do not happen in one leap; they proceed through a series of simple steps. Add those steps up correctly and they must equal the overall reaction, with the fleeting intermediates canceling out.

§1

A reaction, step by step.

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A reaction mechanism is the sequence of elementary steps by which a reaction actually occurs. Each step is a single molecular event, and together they describe the real pathway from reactants to products.

The elementary steps must sum to the overall balanced equation. When you add them, species that are produced in one step and consumed in a later one — the intermediates — cancel out, leaving the overall reaction.

An intermediate is formed and then used up within the mechanism, so it does not appear in the overall equation. (A catalyst, by contrast, is present at the start, consumed early, and regenerated — also absent from the overall equation, but for the opposite reason.)

UNIT 5 TOPIC 5.7 • INTRODUCTION TO REACTION MECHANISMS MECHANISM MAP A mechanism is a sequence of elementary steps that adds up to the overall reaction. Elementary steps each step is a single molecular event Step 1 A + B → C + I Step 2 I + D → E Step 3 E → F + G Intermediates I and E are highlighted. Overall reaction add all steps, then cancel intermediates Sum of all steps: A + B + I + D + E → C + I + E + F + G cancel I and E — each is on both sides A + B + D → C + F + G Intermediates I and E do not appear in the overall reaction. Both I and E are intermediates I formed in step 1, consumed in step 2 E formed in step 2, consumed in step 3 An intermediate is produced in one step and consumed in a later step; it does not appear in the overall reaction. Catalyst vs intermediate Intermediate: made first, then used up Catalyst: used up first, then regenerated CED anchor · Unit 5 Kinetics Summing the three steps and cancelling I and E gives A + B + D → C + F + G — the intermediates never appear. AP Chemistry · Unit 5 · Kinetics
Fig. 5.7.1 A mechanism is the actual sequence of elementary steps by which a reaction occurs. Each step is a single molecular event; the steps sum to the overall balanced equation, with intermediates (formed then consumed) canceling out.
§2

Checking a mechanism.

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Add the steps and confirm they give the overall reaction.

  1. List the elementary steps. Each step is a single molecular event with its own molecularity.
  2. Add the steps together. Combine all reactants and all products of the steps.
  3. Cancel the intermediates. Species produced then consumed cancel; they should not appear overall.
  4. Confirm the overall equation. What remains after canceling must equal the balanced overall reaction.
§3

The pieces you'll meet.

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Steps, sums, and intermediates.

mechanism
Mechanism
The sequence of elementary steps of a reaction.
elementary step
Elementary step
A single molecular event within the mechanism.
sum
Step sum
The steps add up to the overall balanced equation.
intermediate
Intermediate
Formed then consumed; cancels out of the overall equation.
catalyst
Catalyst
Consumed early and regenerated; also absent from the overall equation.
cancel
Canceling
Intermediates cancel when the steps are added.
§4

Worked example: do the steps sum correctly?

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Mechanism. Step 1: A + B → C. Step 2: C + B → D.

Add them. A + B + C + B → C + D.

Cancel the intermediate. C appears as a product of step 1 and a reactant of step 2, so it cancels.

Overall. A + 2B → D. Since C canceled, it is an intermediate and does not appear in the overall equation. If the steps had not summed to a valid overall reaction, the mechanism would be inconsistent.

§5

Mistakes that cost real points.

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Pitfall · 01

"The elementary steps of a mechanism don't have to add up to the overall reaction."

A valid mechanism's steps must sum to the overall balanced equation. If they do not add up (after canceling intermediates), the proposed mechanism is inconsistent with the reaction it claims to describe.

Fix. Add the steps and cancel intermediates; the result must equal the overall balanced equation, or the mechanism is wrong.

Pitfall · 02

"Intermediates appear in the overall equation."

An intermediate is produced in one step and consumed in a later one, so it cancels and does not appear in the overall equation. Seeing a species in a step but not overall is the sign it is an intermediate.

Fix. Identify intermediates as species that cancel when the steps are summed; they are absent from the overall equation.

Pitfall · 03

"An intermediate and a catalyst are the same thing."

Both are absent from the overall equation, but for opposite reasons: an intermediate is formed then consumed, while a catalyst is consumed early then regenerated. A catalyst appears as a reactant in an early step and a product in a later one.

Fix. Distinguish them by order: intermediate = made then used; catalyst = used then remade.

§6

Skill Check.

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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.

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